Abstract:
Edwardsiella tarda is an opportunistic pathogenic bacterium that causes
Edwardsiellosis in cultured and wild fish. It is among the most important bacterial
diseases causing severe economic losses in fish worldwide. This study was carried
out to determine the occurrence and characterize E. tarda in cultured Nile tilapia and
African catfish from selected fish farms in Wakiso district, Uganda. A total of 111
fish samples were collected from 17 fish farms between September 2016 and
February 2017. Clinical and post mortem examination of sampled fish was done
followed by bacteriological examination of the collected internal organs (spleen,
kidney and liver), gills and skin swabs. Identification of the bacterium was done
using conventional biochemical tests, API 20E kits and sequencing of 16S rRNA.
Phylogenetic analysis was done by Neighbor-Joining method in MEGA 7.0.2
against the 16SrRNA gene sequences retrieved from the GenBank. The isolate was
screened for presence of selected virulence genes by polymerase chain reaction
(PCR). From this study, one isolate from O. niloticus was identified and confirmed
to be E. tarda by the 16S rRNA sequencing. The isolate gave an identity rate of
99.9% to other members of E. tarda on comparison with known 16S rRNA
sequences in the GenBank database. In phylogenetic analysis, the isolate did not
cluster with any of the E. tarda isolates suggesting a distant relationship with the
isolates whose sequences were included in this study. Six virulence genes were
detected in the isolate that enhance bacterial survival and pathogenesis in the host
including; CitC, muk, gadB, katB, esaV and fimA. Although the study confirmed
only one E. tarda isolate, the isolate tested positive for several virulence genes
indicating their potential to cause disease in fish and since the bacterium is of public health importance, awareness should be created amongst fish farmers and
stakeholders to take precaution to avoid disease outbreak.
